The separation and enrichment of air by the use of either rate or equilibrium selective sorbents has been practiced for some time. Nitrogen selective sorbents, as typified by ion-exchanged zeolites, are nitrogen selective at equilibrium and have been used in pressure swing adsorption (PSA) processes. Similarly, carbon molecular sieves (CMS) are used for air separation by PSA processes and rely on a rate selectivity for oxygen. Adsorbents that are oxygen selective at equilibrium are preferred for many applications since cycle times for PSA processes are not constrained as typically required for rate selective sorbents.
Transition element complexes (TECs) are one class of materials known to react reversibly at or below ambient temperatures without breaking the O.dbd.O double bond. The use of TECs to selectively remove oxygen from its mixtures with other gases has been disclosed for solutions of TECs, for solid-state TECs or slurries of said solids, for TECs supported physically on solid supports, for TECs incorporated in zeolites and for TECs bound chemically to physical supports. Each of the known approaches for the use of TECs have been beset by one or more of the following problems: (1) insufficient oxygen capacity, (2) slow reaction rates, (3) decreasing reactivity with time, and (4) a metal ion: oxygen binding ratio of 2:1 (.mu.-peroxo). Due to these problems, none of such TEC systems has yet been employed in commercially acceptable embodiments for air separation or oxygen removal from gas stream applications.
Extensive literature reports exist describing the reversible oxygenation of TECs having tetradentate ligands, particularly in solution. These materials require an exogenous base (e.g. a molecule or ion, added as a separate component, with a site or sites capable of coordinating to the metal center by electron donation) such as pyridine. The use of an exogenous base is necessary for TECs based on tetradentate ligands in order to provide the five-coordinate deoxy TEC sites required for superoxo binding.
One class of TECs is referred to as "protected" TECs. These use ligand superstructures referred to as "caps", "picket-fences", and "bridges" to sterically inhibit m-peroxo binding and to provide a permanent void on one face of the TEC that serves as an oxygen interaction site. Examples of such ligand systems include porphyrins, cyclidenes, and Schiff bases. Unfortunately, the number, complexity, and yields of the synthetic steps required to make TECs based on these superstructured ligands results in costs that are prohibitively high for many applications. In addition, the high molecular weights inherent in superstructured TECs restrict the oxygen loadings and storages that are achievable. Finally, oxygen interaction rates are slow for known non-supported solid forms of protected TECs due to intracrystalline diffusion.
More recent reports disclose TECs having tetradentate ligands containing substituents capable of inhibiting .mu.-peroxo dimer formation in solution, that can be prepared with relative ease and have relatively low molecular weights. The substituents in these systems are typically attached at a single-point. These materials require exogenous donors to provide five-coordinate deoxy TEC sites, and do not show sufficient oxygen uptake in the solid phase for commercial application.
Reversible oxygenation of TECs having pentadentate ligands in dilute solution is also known. These include examples having substituents that inhibit .mu.-peroxo dimer formation, and where the ligand structure and donors are intramolecular. To date, none of the known materials have been found to react reversibly with oxygen in the solid state.
The preparation of coordination polymers based on discrete molecular TECs incorporating sites capable of intermolecular donation has also been described. To date, however, none of these examples have been found to react reversibly with oxygen in the solid state.
Solid state TECs offer several advantages over those in dilute solution as the latter materials have problems which have hampered commercial development such as solubility, solvent loss, viscosity, and TEC lifetime.
The ability of transition element centers in some solid state TECs to undergo a reversible interaction with oxygen is known, and the use of supports to disperse or distribute oxygen selective sites derived from discrete molecular TECs to form oxygen selective sorbents has been described. Unfortunately, the reported examples where TECs are dispersed on or within a support, within a polymer, or as an integral part of the polymer, contain insufficient oxygen selective sites for practical use. As an example, Basolo et al ("Reversible Adsorption of Oxygen on Silica gel Modified by Imidazole-Attached Iron Tetraphenylporphyrin", J. Amer. Chem. Soc., 1975, 97, 5125-51) developed methods to attach iron porphyrins to silica gel supports via an axial donor. While these demonstrated a substantial improvement in stability relative to solution systems, the TEC content reported was less than 0.1 mol/kg.
Hendricks, in "Separation of Gases via Novel Transition Metal Complexes," Report Number NSF/ISI87101, Aug. 21, 1987 discloses attempted to prepare oxygen selective sorbents based on TECs by intermolecular donation using peripheral ligand sites.
However, it was concluded that the materials tested did not "rapidly and efficiently adsorb oxygen" and that this apparently was due to unfavorable molecular packing.
Another series of materials having oxygen selectivity at equilibrium includes cyanocobaltate materials such as lithium pentacyanocobaltate solvates. While gas separation processes which utilize these materials have been disclosed, ranges of composition are restricted, and an ability to optimize performance by adjusting isotherm shapes is limited.